Available online at www.banglajol.info Bangladesh J. Sci. Ind. Res. 52(3), 167-176, 2017
Influence of the micro-structural factors upon thermal and mechanical properties of various bag leathers A. K. Mondal and P. K. Chattopadhyay* Government College of Engineering and Leather Technology, Salt Lake, Kolkata-700106, India
Abstract Four different bag leathers, such as, Sheep Bag Leather (SBL), Buffalo Vegetable Tanned Leather (BVTL), Cow Drum Dyed Dry Milled Leather (CDDDML), and Cow Crocodile Print Leather (CCPL), were processed by different methods from respective wet-blues of Indian origin. Thermal degradation profile and mechanical properties of the samples were evaluated, and crosslink densities of each sample were measured by fitment of Mooney-Rivlin equation on stress-strain plots. Morphological characteristics (e.g. fibre structure, fibre thickness, splitting etc.) of all the specimens were investigated by image analyses of SEM photomicrographs. The highest crosslinking density for BVTL was attributed to its higher fibre and fibril thicknesses coupled with rigorous retanning by vegetable tannins and syntans. Embossing at higher temperature and pressure reduced CCPL’s elongation-at-break value and hence stretchiness possibly due to the development of set properties within the CCPL matrix. SBL was noted to contain huge void spaces that increased its stretchiness, and melamine formaldehyde syntans showed pronounced influence in increasing the thermal resistance of both CCPL and CDDDML. Keywords : Crosslink density; SEM; TGA; Fibre thickness
Introduction Leather processing from animal hides and skins is basically the modification of fibrous protein, which requires a series of unit operations and processes. Different types of ingredients (i.e. natural/ mineral/ synthetic tanning/ retanning agents, fatliquoring agents, binders etc.) are incorporated during different stages of leather processing to achieve certain characteristics of leather. Mechanical behavior of any tanned leather, a key attribute from the end user point of view, depends on the leather structure, layer type (i.e. grain or corium), topographical zone, nature and size of defects, sort and age of cattle, tanning and finishing process, etc. (Milašiené and Bubnyté, 2007; O'leary and Attenburrow, 1996). In fact, mechanical properties of leather are dependent upon a large number of structural and molecular factors (e.g. molecular weight, cross-linking and branching of collagen fibre, crystal morphology, fillers, molecular orientation, phase separation, plasticity etc.). The collagen fibril diameter and tear strength were noted to be correlated in bovine leather, with stronger leather having thicker fibrils (Wells et al., 2013). In addition, numerous environmental or external factors, like temperature, pressure, type of deformation, moisture content, stress and strain amplitude and rate, thermal history etc., can influence mechanical properties (Nielsen and Landel, 1994; Arumugam et al., 1992; Bajza and Vrcek, 2001). In fact, mechanical behavior of a material is adjudged by the response of the material against the *Corresponding author e-mail:
[email protected]
applied external forces (Hosford, 2005). Among the mechanical properties, stress-strain behavior can demonstrate the modulus, strength and toughness of the material. In this regard, tensile strength is an important physical criterion that is closely related to the durability of a leather article. Moreover, resistance against thermal degradation is directly linked with the durability and stability of a material. Both modulus and heat resistance of collagenous materials are directly proportional to the degree of cross linking (Charulatha and Rajaram, 2003). Indeed, the thermal degradation of a densely crosslinked sample should be slower, and should take place at higher temperature in comparison to samples having fewer cross-links. Many researchers already established effects of poly-acrylates/ polyacrylic acids in improving thermo-mechanical behavior of chrome tanned leather (Nashy et al., 2010; Christner et al., 1990). Beneficial effects of titanium, melamine-formaldehyde resin, resorcinol tanning agent on chrome free leather have been reported (Zengin et al., 2012). In the past, various workers reported thermal denaturation characteristics of collagen and modified collagen (Zhang et al., 2006). Researchers concentrated on understanding the effects of pretanning processes on collagen structure and reactivity (Brown et al., 2013). Moreover, effects of retanning agents on dry heat resistance of leathers were studied extensively by Wu et. al., and they reported that
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the retanning agent possessing lower hydrophilic character provided the higher dry heat resistance in leather (Wu et al., 2013). Recently, visco-elastic behavior of leather was pursued by several workers (Langroudia et al., 2012; Liu et al., 2007). Few workers reported the effect of biaxial stretching upon the fibre orientation and associated tensile modulus, and they noted that the fibre reorientation factor is a major cause behind change in the tensile modulus (Sturrock et al., 2004). Such biaxial stretching altered area retention of dried leather in addition to the alteration of mechanical properties (Liu et al., 2010). Most recently, significant findings were derived based on morphological studies of both fibre and fibre bundles of leather in two different modes (e.g. light transmission mode and light reflective mode) by metallographic sample preparation (Zhang et al., 2014). Therefore, dependencies of thermal and mechanical properties on the micro-structural attributes are important factors from the perspective of leather article end users. In the present work, mechanical and thermal characteristics of four different bag leathers, prepared from different wet-blues of Indian origin based on different standard recipes, have been attempted to be correlate with the morphological attributes, such as, structure, thicknesses of both fibre and fibre bundle and the role of the different structural and cross-linking components present inside the leather matrices. Materials and methods Different wet blues [i.e. cow wet blue (area = 13 sq. ft., thickness = 1.1-1.2 mm, weight = 1400 g), buffalo wet blue (area = 12 sq. ft., thickness = 1.2-1.3 mm, weight = 1500 g), and sheep wet blue (area = 5 sq. ft., thickness = 0.8-0.9 mm, Soak back
Neutralization
Fixing
weight = 400 g)], required for leather processing, were supplied by M/S Sheong Shi Tannery, Kolkata. All the auxiliaries (e.g. fatliquors, syntans, wetting agent, dye, preservative etc.) utilized during leather processing were provided by Viswaat Chemicals Ltd. and Schill & Seilacher GmbH & Co. (Table I). Other common chemicals were purchased from local suppliers. Preparation of samples Different leather samples [i.e. Sheep Bag Leather (SBL), Buffalo Vegetable Tanned Leather (BVTL), Cow Drum Died Dry Milled Leather (CDDDML), and Cow Crocodile Print Leather (CCPL)] were manufactured following the generalized unit operations for leather manufacturing from wet blue to crust (Fig. 1), based on the standard recipes (Table II). Following are the basic purposes for different unit operations: (a) soak back (to rehydrate the wet blue), (b) rechroming (to increase chromium content), (c) basification (to fix the added chromium compound), (d) acid bating (to open up the fibre bundles) (e) neutralization (to remove free acids and to reach desired pH level for subsequent operations), (f) dyeing (to impart color), (g) re-tanning (to reinforce and fill the substrate), (h) fatliquoring (to impart lubricating effect within fibrous networks) (i) fixing (to improve adhesive forces among the dye molecules and the collagen substrate), (j) top dyeing (to get desired color), (k) mechanical operations (to get required physical properties and appearance), (l) finishing (conducted only in case of CCPL to provide the desired embossing effect). In this regard, rechroming and acid bating operations were omitted in case of CCPL. In addition, CCPL was treated by dye solution, binder and silicone emulsion followed by embossing with crocodile print at 150⁰C and 250 atmospheric pressure.
Rechromming
Basification
Pretan and prefat
Retan and Dye
Top Dyeing
Drying
Acid bating
Fatliquoring
Mechanical operations
Finishing*
* Operation conducted only for CCPL Fig. 1. Schematic representation of generalized crusting process for various samples
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Table I. List of leather auxiliaries showing manufacturers and technical characteristics Sl. No. Name of the auxiliary
Technical characteristics
Manufacturer
1
Vicalan NT953
Fatty alcohol polyglycol ether
VCL1
2
Vinsul VM70
Sulphited fish oil
VCL
3
Vicatan CRS
Technically pure chromium combined with aromatic sulfonic acid condensates
VCL
4
Vicatan VKM
Naphthalene sulfonic acid condensate
VCL
5
Vicatan HUM
Aqueous dispersion of an acrylic copolymer
VCL
6
Vegrassol CS14
Aliphatic hydrocarbons and alcohols sulfonated by special methods
VCL
7
Vicatan VOS
Phenol condensation products
VCL
8
Vicatan FV6
Based on melamine resin
VCL
9
Vicatan FS3
Based on Dicynadiamide.
VCL
10
Wattle GSP200
Mimosa GS powder(double bleach)
11
Quebracho
Quebracho extract
12
Chestnut Veg CN 200
Chestnut extract
13
Vicatan DL
Aromatic sulfonic acid condensation product
VCL
14
Vicastol SP
Based on functional substitute of sperm oil and hydrocarbon emulsified
VCL
15
Vicastol 94V
Aliphatic hydrocarbons and alcohols sulfonated by special methods
VCL
16
Vicatan NP
Base on synthetic neatsfoot oil combined with special softener
VCL
17
Veprovol VA
Combination of natural phospholipids with synthetic softener
VCL
18
Vicastol SL
Blend of specially treated lanolin with saturated hydrocarbon
VCL
19
Perfectol HQ
Aqueous emulsion of high molecular weight paraffines in combination with highly effective waterproofing additives
20
TolcideC-30
Based on ThiocyanomethylthioBenzothiazole (TC MTB)
21
Nitotan Red RSNI
C.I. Acid Red 114 (azo dye)
22
Vicosin 2255
Aqueous polyacrylate dispersion
23
Texzyme HS
Anti wrinkle and high softening enzyme based on protease
24
Suprinil HK Dark Brown 01 liquid
Highly concentrated solutions of metal complex dye in water and water miscible solvent
1Viswaat
Schill&Scillecher
Nitin Dye Chem Pvt. Ltd. VCL Tex-Biosciences Pvt. Ltd. BASF India Ltd.
Chemicals Limited
Characterization
Mechanical properties
Scanning Electron Microscopy (SEM)
The mechanical behavior was investigated by the tensile test. Initially, all the samples were conditioned at 25oC and 65 ± 2% R.H for 48 h. The usual dog-bone shaped specimens for the measurement of the mechanical properties were punched out from the crusts with ASTM Die-C. The measurement as per ASTM D-2209 standard was carried out in a Hioks-Hounsfield UTM-H10 KS (Test Equipment, Surrey, England) maintaining a crosshead speed of 500 mm.min-1 at
The morphological structures of gold coated fractured surfaces for each leather samples were observed under SEM (ZEISS EVO-MA 10) at 20KX magnification. Thereafter, thicknesses of collagen fibres and fibrils were measured by analyzing photomicrographs by image analyses software, Image J, NIH, USA.
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25oC. For each sample, the averages of four test specimens were reported. The force-elongation curve was plotted with Lab Tensile software, from which the tensile strength (TS) and elongation-at-break (EB) were calculated. In each case, the error corresponding to tensile modulus, TS, EB measurement was limited to ± 1 %, ± 2 %, and ± 2 %, respectively Crosslink density of samples were measured based on the Mooney-Rivlin equation (equation 1) which is suitable to measure the degree of crosslinking via fitment of the data obtained from stress-strain results (Tyagi et al., 2014). σav C2 = C1+ .......(1) 2(λ-λ-2 λ
Here, C1 and C2 are constants. The values of C1 and C2 were determined by fitting the plot of σav -2 against 1 to the λ 2(λ-λ Mooney-Rivlin equation. The value of C1 was used to determine crosslink density by equation 2: 2C1 NC = ........................(2) RT
EHT = 15.00 kV WD = 7.5 mm
CRNN (CU)
Signal A = SE1 Mag = 20.00 KX
Date : 25 Feb 2015 Time : 13:12:27
ZEISS
(c)
1 ųm
Thermo Gravimetric analyses (TGA) The thermal stabilities of samples were studied by thermogravimetric analyzer (model: Pyris 6 TGA manufactured by Perkin-Elmer instruments, the Netherlands). About 5–10 mg of samples were heated from ambient temperature to 600oC in the nitrogen atmosphere at the heating rate of 20oC.min-1, and the weight loss vs. temperature plot along with the derivative thermograms (DTG) were recorded in the software. The error in measurement was limited to ± 2 %. Results and discussion SEM Among the SEM photomicrographs, prominent and thick fibre bundles can be observed in case of BVTL (Fig. 2a), which can be reflected in the measured thickness of the fibre
(b)
(a)
1 ųm
Here, NC is the crosslink density, R is the gas constant (8.314 J.deg-1.mol-1) and T is the absolute temperature.
1 ųm
EHT = 15.00 kV WD = 7.5 mm
CRNN (CU)
Signal A = SE1 Mag = 20.00 KX
Date : 25 Feb 2015 Time : 12:48:43
ZEISS
EHT = 15.00 kV WD = 7.5 mm
CRNN (CU)
Signal A = SE1 Mag = 20.00 KX
Date : 25 Feb 2015 Time : 13:02:47
ZEISS
(d)
EHT = 15.00 kV WD = 8.0 mm
CRNN (CU)
Signal A = SE1 Mag = 20.00 KX
Date : 25 Feb 2015 Time : 12:58:17
ZEISS
1 ųm
Fig. 2. SEM photomicrographs of (a) BVTL, (b) SBL, (c) CDDDML and (d) CCPL at 20 KX magnification
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Table II. Comparative recipes of different bag leathers Unit Operations Soak back
Rechroming
Basification Acid bating Neutralization
Pretannning
Retanning & dyeing
Ingredients Water
SBL
150
% of ingredients (w/w)
BVTL
150
CDDDML 150
150
Vicalan NT953
0.25
0.25
0.25
0.25
Oxalic acid
0.5
0.5
0.5
3
Water
100
100
100
Formic acid
0.75
0.75
0.75
Basic chromium sulfate
4
4
4
60 10+20
1:10 Dilution
Vicatan CRS
2
2
2
1
1
1
Sodium formate
0.5
0.5
0.5
Sodium bicarbonate
0.5
0.5
0.5
60
1:16 Dilution
Vicazyme FL
0.5
0.5
0.5
120
L/O/N (except CCPL)
60
Water
100
100
100
100
Sodium formate
1
1
1
1
Vicatan VKM
1
2
1
2
Ammonium bi carbonate
1.25
0.25
1.5
Water
60
60
60
60
Vicatan HUM
2
4
3
3
Vicastol CS14
1
1
2
Veprovol VA
1
1
1
Vicatan VOS
2
4 2
2
Wattle
2
5
Quebracho
2
4
30
1:3 Dilution
20 4 8
4
8
4
8
Dye
3
3
3
3
Vicatan DL
1
1
1
1
100
100
100
100
Veprovol VA
2
2
3
Vicastol NP
2
2
Vinsul VM70 Water
Neutralization pH = 5.4-5.5
2
Vicatan FS3
Chestnut
2
Perfectol HQ
120
L/O/N ( for CCPL)
7
Vicastol SL
Fixing
Remarks
Vinsul VM90
Vicatan FV6
Fatliquoring
CCPL*
Time (min)
2 2
3
Vicastol 94V
2
2
2
Vicastol SP
2
2
2
2 2
Vicalan NT953
0.2
0.2
0.2
0.2
Preservative
0.2
0.2
0.2
0.2
Formic acid
2
2
2
2
90
1:5 Dilution
3*10+30
1:10 Dilution
Top dyeing Water
150
Dye
1
Formic acid
1.5
20 3*10+20
*CCPL has sprayed twice cross with a mixture of 100 parts Dye solution, 150 parts Vicosin 2255 and 750 parts Water followed by a cross spray with 10 parts HM 12521 and 1000 parts water before print (at 1500C and 250 atm. pressure).
bundles (Table III). In fact, thicknesses both fibre and fibre bundles are the highest for BVTL (Table III). On the other hand, SBL shows lots of cavities among its fibre bundles,
which may be created owing to removal of natural fats and oils from the leather during degreasing operations (Fig. 2b). Presence of ruptured fibres with very few void spaces is the
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Table III. Fiber and fiber bundle thicknesses of various samples Sample
Fiber bundle Thickness (µm) Range Average
Fiber Thickness (µm) Range Average
SBL
40.35 - 73.82
55.29
0.66 - 2.95
1.87
BVTL
79.69 - 126.00
105.27
2.67 - 4.85
3.67
CCPL
50.77 - 121.34
85.36
2.65 - 4.56
3.42
CDDDML
62.13 - 103.83
87.58
2.15 - 4.86
3.66
characteristics of CCPL, which can be expected as embossing is associated with application of huge compressive force that can reduce the distances among fibres and fibrils (Fig. 2d). The applied embossing force may be responsible behind the flat look of CCPL wherein fibres and fibre bundles may have stuck together. In this regard, considerable splitting of fibre bundles can be observed in CDDDML which can be due to separation of fibrils from the fibre bundles owing to considerable reduction in inter-fibril cohesive forces by the action of enzymes, added lubricants and applied mechanical forces during milling operation (Fig. 2c). Mechanical properties Comparative stress-strain plots of the samples showing moduli are noted to be in the following order: BVTL > CCPL > CDDDML > SBL (Fig. 3). Respective inherent strengths of the raw materials can be decisive factor behind the tensile moduli as the inherent strength of buffalo hide is the highest whereas sheep skin’s inherent strength is the lowest, and the inherent strength of cow hide is in between those of buffalo hide and sheep skin. Moreover, larger quantity of acrylic copolymers based crosslinking agents and vegetable tanning agents used during manufacture of BVTL can enhance its tensile modulus. In fact, in the presence of acrylic polymer, the collagen supra-molecular structures can be opened up through electrostatic forces as these acrylic polymers contain active sites for charge interactions. Such an opening up is likely to facilitate vegetable tannin molecules to diffuse into the collagen matrix and bring about improved stability to BVTL (Madhan et al., 2002). Interestingly, CCPL shows comparatively higher modulus than that of CDDDML despite the absence of rechroming step during manufacture of CCPL (Table II). Higher tensile modulus for CCPL can be due to the following reasons: a) In contrast to CDDDML, acid bating is not executed in CCPL (Table II), and hence the degradation of collagen network by bating agents or enzymes in CCPL is not possible. b) Incorporating higher proportion of neutralizing syntan in
CCPL may ensure better dispersion of vegetable tannins, syntans etc. in the matrix. c) Applying higher percentage of fatliquoring agents (e.g. synthetic, lanoline, lecithin based oils) in CDDDML can reduce its elasticity as compared to CCPL. d) Usage of substantially huge quantity of vegetable tanning materials, like, wattle, chestnut, quebracho in CCPL effectively reinforce the hide matrix, which is not possible in case of CDDDML. e) The embossing or printing operation at high temperature (150oC) and pressure (250 psi) conducted on CCPL can generate set properties in the matrix which in turn may enhance rigidity and immobilization of the collagen chains against the applied tensile force. f) Milling operation executed on CDDDML can produce softness via breaking down weaker bonds within the matrix leading to easier relative movement or flow of the collagen chains or chain segments in the direction of the applied tensile force. EB profiles for various samples are noted to following order: SBL > CDDDML > BVTL > general, EB of a three dimentional polymeric system like leather mainly depends upon the factors:
be in the CCPL. In composite following
a) orientation of fibres and fibrils along the direction of the applied force, b) availability of void spaces inside the leather matrix, c) presence of lubricating or plasticizing agents (e.g. oil, wax etc.), reinforcing fillers, crosslinking or tanning agents (e.g. vegetable tannins, mineral tanning agents, synthetic tanning agents etc.) In case of SBL, the highest EB value among all the samples might be due to presence of greater extent of void spaces owing to removal of inter-fibriliar materials during the pre-tanning stage. Moreover, in the sheep skin, the presence
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Mondal and Chattopadhyay 32 SBL
28
BVTL CDDDML
24
CCPL
Stress, MPa
20 16 12 8 4 0
0
20
40
60
80
100
120
140
Strain, %
Fig. 3. Tensile properties of various samples of strata of fat cells in the corium affects the interlayer cohesive forces between grain and corium and thus affects the mechanical properties of the SBL (Fig. 3). On the other hand, low EB values for both BVTL and CCPL can be ascribed to lack of void spaces in the matrix together with steric hindrances imposed by the bulky vegetable tannin molecules around the collagen chains (Table V). As a result, the conformational rotations of protein macromolecules may be hindered, which eventually can resist the elongation as well as orientation of macromolecular collagen chains in the direction of applied tensile force. The tensile strengths of four different types of leathers are in the following order: BVTL > SBL > CDDDML > CCPL. The lower tensile strengths for both CDDDML and CCPL may be ascribed to their reduced toughness as these materials are less capable to absorb and tolerate the applied strain energy. This may be possible because of deficiency in structural organization that can be responsible behind stress concentration which should facilitate faster crack initiation leading to ultimate rupture of the specimens. Owing to possible development of thermoset properties at higher temperature and pressure, mechanical properties have been affected in CCPL including the impairment of tensile strength. The highest tensile strength for BVTL is due to its favorable structural parameters like the lower angle of weave greater number of fibrils in a fibre, increased thickness of fibre bundle as well as fibre etc. (Table III). This is because, a thick fibre should be made of greater number of thick fibrils, and several studies show that collagen
fibrils of larger diameter are normally present in the stronger tissue (Wells et al.,2013). Interestingly, better tensile strength has been recorded in case of SBL as compared to both the cow leather samples (i.e. CDDDML and CCPL), which may be due to easier orientation of fibrils and fibres in the direction of applied force as huge void space is available in SBL (Mondal and Chattopadhyay, 2016). In this regard, it is well known that strength of leather depends on orientation of fibres, and the comparatively stronger leather possesses the higher orientation index than that of the weaker leather (Basil-Jones et al., 2011; Basil-Jones et al., 2013). Thermal properties Initially, the thermal degradation of cow leathers (i.e. CDDDML and CCPL) occurs in a rapid fashion as compared to both SBL and BVTL (Fig. 4 and Table IV). In fact, upto 150⁰C, the highest degradation (~ 18%) has been registered for CDDDML among all the samples, which can be ascribed to the evaporation of moisture content as well as the vaporization and removal of fats and oils in a huge quantity that have been incorporated earlier during fatliquoring of CDDDML. Interestingly, within 150-350⁰C, thermal degradations of both CDDDML and CCPL have been arrested, which are realized from the absence of DTG peaks at ~280-285⁰C (Table IV), and thus both the samples demonstrated better resistance against thermal degradation in comparison to both SBL and BVTL. The possible reason can be the usage of melamine-formaldehyde resin (4%) during
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SBL
100
BVTL CDDDML CCPL
80
Weight, %
60
40
20
0
0
100
200
300
400
500
600
Temperature,
o
C
Fig. 4. Thermal properties of various samples processing of both CDDDML and CCPL (Table II), and the melamine-formaldehyde resin decomposes within 345-347⁰C (Bann and Miller, 1958). In fact, the added melamine-formaldehyde resin may have delayed the decomposition process resulting in huge amount of residue formation (i.e. ~20%) upto 600⁰C for both the cow samples (i.e. CDDDML and CCPL), whereas residue contents for both SBL and BVTL are remarkably low (i.e. 6.4% and 5%, respectively). This can be possible via reaction between olated/ oxolated Cr3+ complex and amines of melamine formaldehyde. In this context, interaction between Cr3+ complex and ammonia, pyridine, amines were reported, wherein axial water molecule of Cr3+ complex was replaced
by ammonia, amines etc (Heier et al., 2014). In fact, this reaction was utilized to produce Cr3+ complexes that can act as sensors to detect the presence of amines (Heier et al., 2014). Nashy et al. (2012) confirmed possible interactions between amines of collagen and Cr3+ complex in the chromium tanned leather. Thus, enhanced thermal stability imparted by melamine formaldehyde within cow samples can be expected. On the other hand, SBL showed the lowest overall thermal resistance among all the bag leathers, which can be ascribed to its lowest inherent fibrous strength, the higher angle of weave, lower fibril and fibre thickness (Table III) etc. Moreover, during processing of SBL, the lowest quantity of syntan has been used, with the complete absence
Table IV. DTG peaks and respective MRDs1of different samples 1st Peak
MRD
2nd Peak
MRD
3rd Peak
MRD
4th Peak
MRD
(°C)
(%/ min)
(°C)
(%/ min)
(°C)
(%/ min)
(°C)
(%/ min)
SBL
87.02
3.4
285.52
6.8
343.20
8.0
467.21
5.7
BVTL
90.0
2.5
280.1
4.1
348.8
8.9
467.65
5.6
CDDDML
92.1
4.1
Absent
Absent
349.06
9.6
Absent
Absent
CCPL
90.2
4.7
Absent
Absent
352.54
6.8
480.1
3.6
Sample
1 MRD = Maximum Rate of Decomposition
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Mondal and Chattopadhyay of melamine type syntans in the recipe (Table II). Therefore, SBL shows the lowest crosslinking density (Table V), evaluated by Moony-Rivlin equation. Beyond 250⁰C, thermal degradation profile for BVTL is noted to be slightly better than SBL (Fig. 4), which may be owing to its higher crosslinking density reflected in the superior tensile modulus (Table V), as comparatively higher extent of retanning agents including vegetable tannins (i.e. wattle, quebracho, chestnut) has been incorporated in BVTL
of tanning agents (e.g. vegetable tannins, acrylic syntans, melamine-formaldehyde syntans), influence of mechanical operations (e.g. milling, embossing), thickness and orientation of fibres and fibre bundles. Higher thickness of fibres/ fibre bundles with more number of fibrils in a fibre was the key factor behind enormous mechanical strength and crosslink density of bag leather produced from buffalo hide. Addition of melamine-formaldehyde syntans improved thermal resistance of bag leathers manufactured
Table V. Crosslink densities and tensile properties of various samples Sample name BVTL SBL CCPL CDDDML
C1
NC = 2C1/RT, mol.m-3
Tensile strength, MPa
Elongation-at-break (%)
39.23 7.33 16.83 11.53
0.031668 0.005917 0.013586 0.009307
30.0 15.87 9.5 12.92
53.75 137.80 41.50 75.50
(Table II). These vegetable tannins comprise of numerous phenolic –OH groups which can be invloved in H-bond formation within BVTL (Wu et al., 2013). Within 250-350⁰C, drastic thermal degradation has been noted for SBL followed by BVTL, which is reflected their multiple degradation peaks (2 nos.) within that particular temperature range (Table IV). Such degradation in a rapid fashion may be possibly due to deamination of collagen as deamination might be the dominant decomposition process within 250–350°C. Ammonia originates from deamination reactions of collagen involving free –NH2 groups from arginine and lysine (51 and 25 residues/1,000 residues) and peptide –NH– groups (Cucos and Budrugeac, 2014). In this regard, SBL should contain the highest extent of free –NH2 groups as SBL manufacture is associated with the usage of lowest quantity of vegetable tannins (Table II). In fact, application of higher amount of vegetable tannins reduces the availability of free –NH2 groups. This is because, functional groups of tannins interact mainly with amine (–NH2) or imine (–NH–) groups of collagen. At a higher temperature range, a further reduction in mass may be attributed to thermal decomposition/ volatilization of the peptide chains, backbones, glycosaminoglycans leading to ultimate conversion into elemental carbon. Conclusion It can be concluded that both mechanical and thermal properties of a bag leather depend largely on inherent strength of the raw material, extent, type and combination
from cow hides whereas embossing at high pressure and temperature reduces elongation-at-break of bag leather via generating set properties. References Arumugam V, Naresh MD, Somanathan N and Sanjeevi R (1992), Effect of Strain Rate on the Fracture Behavior of Collagen, J. Mater. Sci. 27: 2649-2652. Basil-Jones MM, Edmonds RL, Cooper SM and Haverkamp RG (2011), Collagen Fibril Orientation in Ovine and Bovine Leather Affects Strength: A Small Angle X-ray Scattering (SAXS) Study, J. Agric. Food Chem. 59: 9972-9979. Basil-Jones MM, Edmonds RL, Cooper SM, Kirby N, Hawley A and Haverkamp RG (2013), Collagen Fibril Orientation and Tear Strength across Ovine Skins, J. Agric. Food Chem. 61: 12327-12332 Bann B and Miller SA (1958), Melamine and derivatives of melamine, Chem. Rev. 58: 131-172. Bajza Z and Vrcek IV (2001), Fatliquoring agent and drying temperature effects on leather properties, J. Mater. Sci., 36: 5265–5270. Brown EM, Latona RJ and Taylor MM (2013), Effects of Pretanning Processes on Collagen Structure and Reactivity, J. Amer. Leather Chemists Ass. 108(1): 23-29.
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Charulatha V and Rajaram A (2003), Influence of different crosslinking treatments on the physical properties of collagen membranes, Biomaterials 24: 759-767. Christner J, Pfleiderer E, Taeger T and Wick G (1990), U S Patent Number: 4,961,750 Assignee: Riihm GmbH, Darmstadt, Fed. Rep. of Germany. Cucos A and Budrugeac P (2014), Simultaneous TG/DTG–DSC–FTIR characterization of collagen in inert and oxidative atmospheres, J. Therm. Anal. Calorim. 115: 2079-2087. Heier P, Boscher ND, Choquet P and Heinze K (2014), Dual Application of (Aqua) (Chlorido) (Porphyrinato) Chromium (III) as Hypersensitive Amine-Triggered ON Switch and for Dioxygen Activation, Inorg. Chem. 53: 11086 −11095.
52(3) 2017
Nashy EHA, Osman O, Mahmoud AA and Ibrahim M (2012), Molecular spectroscopic study for suggested mechanism of chrome tanned leather, Spectrochim. Acta Mol. Biomol. Spectrosc. 88: 171-176. Nielsen LE and Landel RF (1994), Mechanical Properties of Polymers and Composites 2nd Ed. Boca Raton: CRC press. O'leary DN and Attenburrow GE (1996), Differences in strength between the grain and corium layers of leather, J. Mater. Sci. 31: 5677-5682. Sturrock EJ, Boote C, Attenburrow GE and Meek KM (2004), The effects of the biaxial stretching of leather on fibre orientation and tensile modulus, J. Mater. Sci. 39: 2481 – 2486.
Hosford WF (2005), Mechanical Behavior of Materials, New York: Cambridge University Press, p 425.
Tyagi PK, Chatopadhyay PK and Mukherjee G (2014), Glutaraldehyde Retannage - a Review and Assessment of the Properties, J. Soc. Leath. Tech. Ch. 98: 99-107.
Langroudia AE, Mirmontahaia A and Vahidzadeh R (2012), Viscoelastic behaviour of treated historical leather with nanocomposite, Proceedings of the 4th International Conference on Nanostructures (ICNS4), 12-14 March, 2012, Kish Island, I.R. Iran, pp 190-192.
Wells HC, Edmonds RL, Kirby N, Hawley A, Mudie ST and Haverkamp RG (2013), Collagen Fibril Diameter and Leather Strength, J. Agric. Food Chem. 61: 11524-11531.
Liu CK, Latona NP, DiMaio GL and Cooke PH (2007), Viscoelasticity studies for a fibrous collagen material: chrome-free leather, J. Mater. Sci. 42: 8509–8516.
Wu C, Zeng Y, Liao X, Zhang W and Shi B (2013), Effect of Retanning Agents on Dry Heat Resistance of Leathers, J. Amer. Leather Chemists Ass. 108(8): 294-299.
Liu CK, Latona NP, Ramos MA and Goldberg NM (2010), Mechanical properties and area retention of leather dried with biaxial stretching under vacuum, J. Mater. Sci. 45: 1889–1896.
Zhang Z, Li G and Shi B (2006), Physicochemical properties of collagen, gelatin and collagen hydrolysate derived from bovine limed split wastes, J. Soc. Leath. Tech. Ch. 90: 23-28.
Madhan B, Muralidharan C and Jayakumar R (2002), Study on the stabilisation of collagen with vegetable tannins in the presence of acrylic polymer, Biomaterials 23: 2841–2847.
Zhang H, Xia Y, Shi L and Li T (2014), Characterization of Leather Structure via Metallographic Sample Preparation, J. Amer. Leather Chemists Ass. 109(1): 1-7.
Milašiené D and Bubnyté K (2007), The influence of fatigue conditions on the mechanical properties of laminated leather and its separate layers, Mater. Sci. 13: 210-213.
Zengin ACA, Crudu M, Maier SS, Deselnicu V, Albu L, Gulumser G, Bitlisli BO, Basaran B and Mutlu MM (2012), Eco-leather: Chromium-free Leather Production Using Titanium, Oligomeric Melamine-Formaldehyde Resin, and Resorcinol Tanning Agents and the Properties of the Resulting Leathers, Ekoloji 21: 17-25.
Mondal A and Chattopadhyay PK (2016), Mechanical and hydrodynamic swelling characteristics of bovine tanned leather post-treated by acrylic and glutaraldehyde tanning agents, Bangladesh J. Sci. Ind. Res. 51(1): 1-12. Nashy EHA, Hussein AI and Essa MM (2010), Retanning Agents for Chrome Tanned Leather based on Emulsion Nano-Particles of Styrene/Butyl Acrylate Copolymers, N.Y. Sci. J. 311: 13-21.
Received: 30 Octorber 2016; Revised: 21 November 2016; Accepted: 28 December 2016.
